This invention relates in general to communication systems, and more specifically to a method and apparatus for assessing transmission performance and using the performance to select a transmission mode.
A frequent goal in designing wireless communications systems is to increase the number of users that may be simultaneously served by the communications system. This may be referred to as increasing system capacity. In multiple access communications systems, including code division multiple access (CDMA) wireless communications systems, the use of adaptive antenna arrays at the base transceiver has been proposed as a method of increasing system capacity.
An adaptive array antenna includes two or more radiating elements with dimensions, spacing, orientation, and an illumination sequence that produce a field from a combination of fields emitted from individual elements that has greater intensities in some directions and lesser field intensities in other directions. An adaptive array antenna assists in increasing system capacity because this field pattern or radiation pattern of the adaptive array antenna, which includes multiple beams or lobes, may be configured such that signals intended for a selected user are in higher-gain antenna lobes pointed in the direction of a selected user, while nulls in the antenna pattern are likely directed to other users. Thus, signals intended for other users in the selected user""s antenna null are not adversely affected by the power intended for the selected user. This increases capacity because one user""s signals are not transmitted with a higher antenna gain to all the other users in the sector or cell where it would degrade all other users"" signals. Other transmission modes such as higher data rates, for increasing system capacity have also been considered.
Unfortunately these various transmission modes or techniques have limitations reflected in varying levels of transmission performance. These limitations and resultant transmission performance are typically related to and can be modeled as functions of channel quality and reliability. For example, a wireless channel can vary dramatically from location to location and will depend on whether the source and target are in relative motion each to the other. Most adaptive array antenna systems and transmission modes using adaptive arrays utilize some form of algorithm, often mobile assisted, to quasi-continually adapt the array to current channel conditions e.g. mobile or portable subscriber position and relative motion in order to maintain or optimize transmission performance. Similarly higher data rate systems tend to be adaptive and require some form of adaptation to current channel conditions. However, the ability of these systems to adapt to changing channel conditions is limited by transport and processing times as well as bandwidth devoted to the adaptation of the system and once this limit is exceeded the systems tend to perform very poorly.
Thus, it should be apparent that a need exists for a method and apparatus for measuring transmission performance and using such measurements for such activities as selecting appropriate transmission modes.